# std::reverse_copy

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< cpp‎ | algorithm

Defined in header `<algorithm>` |  |
template< class BidirIt, class OutputIt > OutputIt reverse_copy( BidirIt first, BidirIt last, OutputIt d_first ); | (1) | (constexpr since C++20)
template< class ExecutionPolicy, class BidirIt, class ForwardIt > ForwardIt reverse_copy( ExecutionPolicy&& policy, BidirIt first, BidirIt last, ForwardIt d_first ); | (2) | (since C++17)

1) Given \(\scriptsize N\)N as std::distance(first, last). Copies the elements from the range `[`first`,` last`)` (source range) to another range of \(\scriptsize N\)N elements beginning at d_first (destination range) in such a way that the elements in the destination range are in reverse order.
Behaves as if by executing the assignment *(d_first + N - 1 - i) = *(first + i)[1] once for each integer i in `[`​0​`,` N`)`.
If source and destination ranges overlap, the behavior is undefined.
2) Same as (1), but executed according to policy.
This overload participates in overload resolution only if all following conditions are satisfied:

std::is_execution_policy_v<std::decay_t<ExecutionPolicy>> is true. | (until C++20)
std::is_execution_policy_v<std::remove_cvref_t<ExecutionPolicy>> is true. | (since C++20)

### Parameters

first, last | - | the range of elements to copy
d_first | - | the beginning of the destination range
Type requirements
- `BidirIt` must meet the requirements of LegacyBidirectionalIterator.
- `OutputIt` must meet the requirements of LegacyOutputIterator.
- `ForwardIt` must meet the requirements of LegacyForwardIterator.

### Return value

Output iterator to the element past the last element copied.

### Complexity

Exactly \(\scriptsize N\)N assignments.

### Exceptions

The overload with a template parameter named `ExecutionPolicy` reports errors as follows:

- If execution of a function invoked as part of the algorithm throws an exception and `ExecutionPolicy` is one of the standard policies, std::terminate is called. For any other `ExecutionPolicy`, the behavior is implementation-defined.
- If the algorithm fails to allocate memory, std::bad_alloc is thrown.

### Possible implementation

See also the implementations in libstdc++, libc++, and MSVC STL.

    template<class BidirIt, class OutputIt>
    constexpr // since C++20
    OutputIt reverse_copy(BidirIt first, BidirIt last, OutputIt d_first)
    {
        for (; first != last; ++d_first)
            *d_first = *(--last);
        return d_first;
    }

### Notes

Implementations (e.g. MSVC STL) may enable vectorization when the both iterator types satisfy LegacyContiguousIterator and have the same value type, and the value type is TriviallyCopyable.

### Example

Run this code

    #include <algorithm>
    #include <iostream>
    #include <vector>

    int main()
    {
        auto print = [](const std::vector<int>& v)
        {
            for (const auto& value : v)
                std::cout << value << ' ';
            std::cout << '\n';
        };

        std::vector<int> v{1, 2, 3};
        print(v);

        std::vector<int> destination(3);
        std::reverse_copy(std::begin(v), std::end(v), std::begin(destination));
        print(destination);

        std::reverse_copy(std::rbegin(v), std::rend(v), std::begin(destination));
        print(destination);
    }

Output:

    1 2 3
    3 2 1
    1 2 3

### Defect reports

The following behavior-changing defect reports were applied retroactively to previously published C++ standards.

DR | Applied to | Behavior as published | Correct behavior
--- | --- | --- | ---
LWG 2074 | C++98 | for each i, the assignment was *(d_first + N - i) = *(first + i)[1] | corrected to *(d_first + N - 1 - i) = *(first + i)[1]
LWG 2150 | C++98 | only one element was required to be assigned | corrected the requirement

1. ↑ 1.0 1.1 1.2 LegacyOutputIterator is not required to support binary `+` and `-`. The usages of `+` and `-` here are exposition-only: the actual computation does not need to use them.

### See also

reverse | reverses the order of elements in a range (function template)
ranges::reverse_copy (C++20) | creates a copy of a range that is reversed (algorithm function object)

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